The present invention relates to endless belts having driving surfaces comprising a high temperature flexible thermoplastic composite, which composite reduces noise, improves dimensional stability and improves abrasion resistance of the belt compared to conventional belt constructions, while minimizing frictional heat effects in such belts, and more particularly to endless toothed belts having an elastomeric belt body portion, a reinforcement member disposed within the body portion, a wear resistant fabric cover, and a high temperature, abrasion- and noise resistant composite arranged along the wear-resistant fabric cover, which composite is substantially free of the belt body elastomer at its surface, and further to a method for producing such belts. In a preferred embodiment, such belts incorporate elastomeric body portions formed from a polyurethane material.
Endless belts, including V-belts, V-ribbed belts, and flat belting, as well as toothed belts such as synchronous or timing belts and the like, are used in a variety of environments. Examples of power transmission belts, including synchronous belts, V-belts, and V-ribbed belts are disclosed in U.S. Pat. Nos. 3,138,962; 3,200,180; 4,330,287; and 4,332,576. Examples of methods for producing such belts are disclosed in U.S. Pat. No. 3,200,180 as indicated above and U.S. Pat. Nos. 3,772,929 and 4,066,732. These patent references are merely examples of the types of belts and state-of-the-art formation techniques thereof.
Toothed belts are put to particularly good use in high temperature, high speed and high load environments, including various industrial and automotive drive systems. In the automotive area, various factors have contributed to the growing demand for such belts which perform under increasingly high loads and temperatures, which are commonly at 120.degree. C. to 140.degree. C., and are expected to react 150.degree. C. or greater. Under high load, high temperature and high speed conditions, it is common for the teeth of endless toothed belts to deteriorate; the severe shearing stresses on the teeth often result in crack generation and tooth loss. Thus, it is known to incorporate a wear-resistant fabric cover element over the tooth and land portions of such belts in an attempt to alleviate this problem. This improvement however has not proved completely satisfactory.
Performance characteristics of endless belts which have become important in automotive original equipment and after-market applications in recent years include minimal frictional heat generation, quiet belt operation, and dimensional stability, as well as high temperature performance. With respect to endless toothed belts in particular, frictional heat generation and heat build-up reduce the efficiency of the belt, and the higher operating temperatures frequently encountered by these belts can reduce belt life considerably by lowering the tear strength and fatigue life of the tooth, or by attacking the bonds between the belt components, e.g., between the elastomer body and tensile cord embedded therein, and between the elastomer body and the wear resistant fabric cover element. Endless toothed belts having elastomeric body portions constructed of a castable elastomer in particular, e.g., some liquid polyurethane elastomers, while offering a number of significant advantages over conventional rubber belting, including a lower susceptibility to flex fatigue, ease of manufacture and improved load life, often run hotter and noisier than comparable rubber belts due primarily to the higher coefficient of friction of these materials compared to the more conventional non-castable elastomers. This is particularly the case with polyurethane-based belts. Thus, the noise problems and frictional heat generation common to endless toothed belts generally are particularly troublesome in polyurethane belt applications. In particular, a polyurethane belt is generally more aggressive as it enters and leaves the sprocket or sheave and builds up considerable heat at the belt-sprocket or sheave interface.
One proposed solution to the noise problem commonly found in conventional belting has been to reduce the coefficient of friction of the driving surface of the belt. What is meant by the term, "driving surface" within this context is that surface of the belt which forms an interface with either a sprocket, in the case of toothed belting, or a sheave, in the case of V-belts or multi-V-ribbed belts. One such approach involves isolating or removing as much of the elastomer as possible from near the surface of the belt where that surface comes in contact with sprocket teeth or flanges. Such an approach is taken in U.S. Pat. No. 3,772,929. Another method for dealing with the noise and frictional heat generation problems in castable elastomer belting is disclosed in U.S. Pat. No. 3,964,328 wherein a layer of elastomer-impervious material, e.g., polyethylene, is utilized during the casting operation and is bonded to one side of a wear-resistant fabric cover element. A further suggestion has been to incorporate a polytetrafluoroethylene (PTFE) layer over the wear-resistant fabric cover element to decrease the effective coefficient of friction of the driving surface of the belt.
Each of these proposed solutions to the belt noise and/or frictional heat problems in polyurethane belts have come at the expense of increased wear and decreased dimensional stability which may nonetheless exacerbate belt noise. Dimensional variations commonly encountered include increased belt length and an alteration in the distance between the center of the load carrying members of a belt and the bottom surface of the land portions between adjacent longitudinally spaced teeth. In toothed belts, belt length changes generally lead to increased slip noise, which is that noise associated with the tangential- or radial sliding or slip between the belt and the sprocket as each tooth enters and exits a corresponding sprocket. For a belt designed and manufactured properly for a given application, as the belt length increases, the magnitude of the slip increases, resulting in increased slip noise.
Where the fabric cover element of a belt has been modified to address noise concerns, such as by incorporating a high number or size of twisted yarns, or by including a relatively low abrasion-resistant or low temperature laminate over the fabric cover which consequently likely flakes off with use or melts at high temperatures, the belt likely experiences a variation in the distance between the center of the load carrying members of a belt and the bottom surface of the land portions between adjacent longitudinally spaced teeth with use in addition to an increase in belt length. Generally, as the low temperature, low abrasion-resistant laminate flakes or melts off of the fabric layer with continued use, the distance between the center of the load carrying members of the belt and the bottom surface of the land portions between adjacent longitudinally spaced teeth would decrease, eventually exposing the fabric layer of the cover element to the sprocket and ultimately causing deterioration of such layer and exposure of the belt elastomer. This variation further causes poor fit between the tooth or land portions of the belt and the corresponding sprocket, hence accelerating both slip and impact noise. Impact noise is that noise which occurs as a result of radial, i.e., normal contact between the tooth or land portions of the toothed belt and the corresponding portion of the sprocket as each tooth or land portion enters it. A substantially pure PTFE layer incorporated on the surface of a wear-resistant fabric cover element, while resulting in a reduced coefficient of friction, exhibits very poor wear resistance, and thus would likely wear off of the belt with use, again leaving the wear-resistant fabric layer exposed.
Known endless belt constructions have not effectively addressed the combined problems of belt noise, frictional heat generation and dimensional instability. In particular, known endless toothed belt constructions having belt body portions formed from polyurethanes, including conventional polyurethanes, polyurethane-ureas and polyureas, do not possess high temperature surface performance capabilities suitable for automotive applications, e.g., for use within automotive engine compartments wherein temperatures frequently range from about 120.degree. C. to about 140.degree. C., and in the near future are expected to reach or exceed about 150.degree. C.
Consequently, there remains a need to produce an endless belt, including an endless toothed belt for use in high temperature applications, which exhibits reduced noise during belt operation, which does not experience significant frictional heat generation, and which otherwise remains dimensionally stable throughout its life. Additionally, it would be advantageous to produce such a belt having belt body portions formed of a polyurethane, such that the belt would possess high temperature characteristics sufficient for use in automotive applications.